qbs build: More steps toward creating a "development installation".

Namely:
    - Install header files.
    - Install imports and modules.
    - Create modules from products and install them.

This is most of what we need. The main thing still left to do is
dealing with paths in Export items. These have to be translated
somehow. (Currently we only copy the Depends items out of the
Export items and ignore everything else.)

Change-Id: I12d49fa31d1c1e05bc77a0e0ce3ec9c78c27192a
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>